JPH03247743A - Sintered alloy steel excellent in corrosion resistance, machinability and mirror finishing property and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture a sintered alloy steel excellent in corrosion resistance, machinability and mirror finishing properties by forming steel powder with specified grain size having a specified compsn. constituted of Cr, Ni, Mo, Mn, S, Se, Te and Fe and thereafter executing specified sintering. CONSTITUTION:Steel powder contg., by weight, 16 to 25% Cr, 8 to 24% Ni, <=10% Mo and <=2.0% Mn, furthermore contg. one or more kinds of free cutting components among 0.02 to 0.3% S, 0.01 to 0.3% Se and 0.01 to 0.3% Te and the balance Fe with inevitable impurities as well as having <=15mum grain size is mixed with a bond and is formed. Next, the bond in this formed body is heated under the reduced pressure or in a nonoxidizing atmosphere and is removed. Preferably, this formed body is heat-treated in lubricant hydrogen or in the air, and the C/O molar ratio therein is regulated to 0.3 to 3.0. Successively, the formed body is heated at 1050 to 1350 deg.C under the reduced pressure of <=30Torr and is furthermore sintered at 1250 to 1350 deg.C in a nonoxidizing atmosphere. In this way, a sintered stainless steel in which the above free cutting components are finely dispersed, having >=92% density ratio and excellent in machinability can be obtd. without deteriorating its corrosion resistance and mirror finishing properties.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a sintered alloy steel manufactured by powder metallurgy that has excellent corrosion resistance, machinability, and specularity, and a method for manufacturing the same.

<Prior Art> In recent years, the field of application of sintered parts using powder metallurgy has shown remarkable growth along with its technical direction -1. Among these, sintered stainless steel has come to be used in a wide variety of fields, and parts with complex shapes are now required. Finishing processing after sintering is essential for manufacturing these complex-shaped parts, and is also important from a process standpoint. However, stainless steel is difficult to machine, and Cr-Ni stainless steel in particular is generally sticky (and has significant work hardening, resulting in severe tool wear and poor machinability. Since a large number of pores remain in the powder sintered body, machinability is poor, and corrosion resistance and specularity are also deteriorated.

In order to solve these problems, for example,
-52001 and free-cutting stainless steel powder as described in Japanese Patent Publication No. 59-25002, or Special Publication No. 54
Free-cutting sintered alloys have been developed, such as those described in Japanese Patent Publication No. 28818 and Japanese Patent Publication No. 55-14862.

The former steel powder is mixed with free-cutting components such as S and Se, and Mn in the base forms MnS and MnSe in the sintered body.
The aim is to improve machinability, and the latter sintered alloy attempts to achieve free machinability by mixing oxide inclusions or lowering the density of the sintered body to some extent. .

However, MnS and MnSe reduce the compressibility of steel powder and are eluted in an environment of artificial sweat, so they have the side effect of significantly reducing corrosion resistance and specularity.

In addition, pores in the sintered body not only deteriorate the corrosion resistance (but also reduce heat conduction, so when cutting under certain conditions, the heat generated during cutting does not escape from the vicinity of the tool, and as a result, This results in increased tool wear.Furthermore, oxide inclusions reduce the compressibility of the steel powder, inhibit sintering, prevent the density of the sintered body from increasing, and reduce corrosion resistance and specularity. be.

<Problems to be Solved by the Invention> An object of the present invention is to improve the machinability of a stainless steel sintered body without reducing its corrosion resistance and specularity.

Specifically, S Se, which reduces corrosion resistance and specularity,
A sintered alloy steel with excellent machinability, corrosion resistance, and specularity, in which free-cutting components such as Te are finely dispersed in a sintered body, and the density ratio of the sintered body to the ingot material is 92% or more, and its A manufacturing method is provided.

Means for Solving the Problems> As a result of various studies, the present inventors came to the following conclusion. In other words, a raw material powder is obtained by adding free-cutting components such as SSe and Te to steel powder so that the average particle size is 15 μm or less, which is molded and degreased, and then vacuum sintered and a non-oxidizing atmosphere are combined. The density of the sintered body relative to the melted material can be made 92% or more, and MnS, MnSe
It has been found that by finely dispersing one or more of MnTe and MnTe, machinability can be significantly improved without impairing corrosion resistance and specularity.

That is, in the present invention, Cr: 16 to 25% by weight, Ni:
8 to 24% by weight, Mo: 10% by weight or less, Mn: 2.0
% by weight or less, C: 0.06% by weight or less and 0:0.5
Including weight% or less, S: 0.02-0.3% by weight, S
e: 0.01~0.3% by weight and T'e: 0.01~
A powder sintered body containing one or more free-cutting components of 0.3% by weight, with the remainder consisting of Fe, J: and inevitable impurities, and the free-cutting component in the sintered body is MnS. , MnSe
, exists as a compound in the form of MnTe, the size of the compound is 15 if m or less, and the density of the sintered body is 92% or more of that of the molten body. The present invention provides a sintered alloy steel with excellent properties.

Further, the present invention has Cr: 16-25% by weight, Ni: 8-25% by weight.
24% by weight, Mo: 10% by weight or less and Mn: 2.0
S: 0.02-0.3 wt%, Se: 0101-0.3 wt% and Te:
Using steel powder containing one or more free-cutting components of 0.01 to 0.3% by weight, the remainder consisting of Fe and unavoidable impurities, and having a particle size of 15 μm or less,
After adding and mixing a binder to this steel powder and molding, the binder in the molded body is removed by heating under reduced pressure and/or in a non-oxidizing atmosphere, and then at a temperature of 1050 to 1350 °C or less and a pressure A method for producing a sintered alloy steel with excellent corrosion resistance, machinability, and specularity, characterized by sintering under reduced pressure of 30 torr or less and further sintering at a temperature of 1250 to 1350°C in a non-oxidizing atmosphere. It provides:

After removing the binder in the compact, if the C10 molar ratio in the compact falls outside the range of 0.3 to 3.0, before sintering,
It is preferable to adjust the C10 molar ratio so that it falls within the above range. The adjustment is preferably carried out by heat treatment under wet hydrogen or heat treatment in the atmosphere.

The present invention will be explained in more detail below.

First, MnS and MnSe in the sintered body.

The size of M n Te was determined because corrosion resistance and specularity are influenced by the size of these compounds, and Cr, Ni, Mo, and M n in the sintered alloy.

The amounts of C, O, S, Se, and Te are specified because they are considered to be elements that greatly affect corrosion resistance.

0r = 16 to 25% by weight The higher the amount of Cr added, the better the corrosion resistance, but if the content is 16%
If it is less than % by weight, a sufficient effect on corrosion resistance cannot be obtained;
Moreover, if it is added in an amount exceeding 25% by weight, there will be no further effect and the cost of the steel powder will increase. Furthermore, the problem of embrittlement of the sintered body due to the sigma phase also occurred, so the upper limit of addition was set at 25% by weight.

N1.8 to 24% by weight Ni is an austenite stabilizing element and is effective in improving properties such as corrosion resistance and toughness of the sintered body.

However, if the content is less than 8% by weight, the effect on stabilizing austenite is small and corrosion resistance is reduced. Also, 2
Adding more than 4% by weight will not produce any more significant effects and will increase the cost of the steel powder, so the upper limit of addition was set at 24% by weight.

MO・10% by weight or less M and o are the most effective elements for corrosion resistance. However, if added in excess of 10% by weight, the toughness of the sintered body will be significantly reduced, so the upper limit was set at 10% by weight.

Mn: 2.0% by weight or less Mn combines with added S, Se, and Te to form MnS, MnSe, and MnTe in the sintered body, thereby improving machinability. However, if the Mn addition exceeds 2.0% by weight, the steel powder will become spheroidized and the density will not improve when it is made into a sintered body, so the upper limit was set at 1.0% by weight.

C・0.06% by weight or less It is generally known that the lower the C content, the better from the viewpoint of corrosion resistance. The reason why the upper limit is set to 0.06% by weight is that if it exceeds this value, Cr carbides will form in the sintered body, creating a Cr-deficient phase, resulting in a significant decrease in corrosion resistance.

There is also the problem that the pores become coarse due to the generation of a liquid phase.

0: 0.5% by weight or less The lower the value of 0, the more densification of the sintered body progresses, resulting in improved corrosion resistance and
Both machinability and specularity are improved. However, 0.
If the content exceeds 5% by weight, sintering will not proceed and a predetermined density will not be obtained, resulting in decreased corrosion resistance, machinability, and specularity.

Sho, 02-0.30% by weight, Se: 0.01-0.
30% by weight, Te: 1 out of 0.01 to 0.30% by weight
One or two of these elements serve as free-cutting components and have the effect of improving the machinability of the sintered body. However, since excessive addition may reduce the corrosion resistance or inhibit the compressibility of the steel powder, the number of added components and their respective upper limits were defined as above. Also,
These elements are M n S and M n S respectively
These compounds are present in the sintered body as compounds in the form of e and MnTe, but these compounds are eluted in an environment of artificial sweat, and the pits form in the pits, which progress pitting corrosion. ”−
If the size of the compound exceeds 15 μm, the remaining bits will grow large, pitting corrosion will rapidly progress, and corrosion resistance will deteriorate. Furthermore, since these compounds have lower hardness than the matrix portion, if their area is large, there is a problem that when mirror polishing is performed, the surface of the sintered body becomes uneven, making it impossible to obtain a desired mirror surface.

The density ratio of a sintered body is a factor that directly affects corrosion resistance, machinability, and specularity. The sintered body has a density ratio of less than 92% to the molten material (molten solidified body of the same alloy steel), and many pores remain in the sintered body, which are plugged and connected to the outside of the chair, so sintering is not possible. The interior of the body is also exposed to a corrosive environment, resulting in a significant decrease in corrosion resistance. Furthermore, when the density ratio is less than 92%, the area of the pores is not only large, but also their shape is irregular, resulting in a decrease in specularity. Furthermore, if pores remain, the thermal conductivity of the sintered body will decrease, and under certain cutting conditions, heat will accumulate near the tool during cutting, resulting in significant tool wear. For the above reasons, the density ratio of the sintered body was set to 92% or more.

Next, as a method for manufacturing such a sintered alloy steel, Cr:
16-25% by weight, Ni: 8-24% by weight, Mo: 10
% by weight or less and Mn: 2.0% by weight or less, further S: 0.02 to 0.3% by weight, Se: 0.01 to
Contains one or more of 0.3% by weight and Te+0.01 to 0.3% by weight as free-cutting components, and the remainder is F.
e and unavoidable impurities, a binder is added to steel powder with an average particle size of 15 μm or less, and after molding, the binder is removed by heating under reduced pressure and/or in a non-oxidizing atmosphere.
Sintering at a temperature of 1050-1350°C and a pressure below 30 torr, followed by sintering at 1250-1350°C in a non-oxidizing atmosphere
It can be obtained by sintering at a temperature of

In the present invention, the amounts of Cr, Ni, Mo, and Mn are specified in order to obtain the above-mentioned sintered alloy steel.

In order to ultimately reduce the size of MnS, MnSe, or MnTe to 15 μm or less, for example, by adding free-machining elements to the pre-alloy, these elements are once dissolved in the steel and then reintroduced. A finely dispersed state is obtained during the precipitation process. In addition, S, Se, and Te are FeS and Fe5, respectively.
The same effect can be obtained even if the powder is mixed in the eFeTe state.

The average particle size of the raw material powder is a major factor that influences the density of the sintered body. The more fine powder is used, the more the sintering progresses and the density of the sintered body increases. However, when using steel powder with an average particle size larger than 15 μm, the density ratio of sintered material to ingot material is 92
%, the pores inside the sintered body are large, and their shape is irregular, making it impossible to obtain the required corrosion resistance, machinability, and specularity. Therefore, the average particle size was defined as 15 μm or less.

Since the particle size of the steel powder used is small, it is difficult to mold the steel powder alone, and even if it is molded, there are problems such as cracks in the molded product and damage to the mold. Therefore, a binder is added to the steel powder for forming. Molding is also possible using commonly used binders such as waxes, resins, or mixtures thereof. The amount of binder added varies depending on the molding method. The molding method may be injection molding, press molding using a mold, or extrusion molding. For example, in mold molding, the amount of binder is about 0.5 to 2% by weight, but in injection molding, it is 10 to 15% by weight. % is required.

After molding, heating is performed under reduced pressure and/or in a non-oxidizing atmosphere to remove the binder. The heating temperature and heating rate are determined by the decomposition and evaporation temperature of the binder.

After removing the binder, C100,3-3.0 in the molded body
If it is not within this range, there is a risk that the appropriate amount of C and O for the sintered alloy steel will not be obtained. ．．
Adjust to 0. Conditioning is carried out by heating the degreased body in wet hydrogen or air.

After that, sintering is performed. At that time, 1050-1350℃
By sintering in a reduced pressure of 30 torr or less and then sintering at a temperature of 1250 to 1350° C. in a non-oxidizing atmosphere, a sintered body with a density ratio of 92% or more can be obtained.

The purpose of the first stage of sintering is to reduce Cr.

If the temperature is lower than 1050° C., the reduction of Cr oxide in the sintered body will not proceed sufficiently, and oxygen will remain, inhibiting subsequent sintering.

In addition, at temperatures higher than 1350°C, Cr excessively evaporates from the surface of the sintered body and deteriorates corrosion resistance, so the upper limit is set to 135°C.
It was defined as 0°C. A reduced pressure is suitable for reducing the Cr oxide, but if the pressure exceeds 30 torr, the reduction of the Cr oxide will proceed (this is why the upper limit was set at 30 torr).

The purpose of the subsequent stage of sintering is to increase the density and homogenize the alloying elements in the sintered body. High densification requires a temperature of 1250°C or higher, and the reason why the upper limit was set at 1350°C is because at higher temperatures, defects such as excessive evaporation of Cr and loss of the shape of the sintered body occur. It is. In addition, the reason why the atmosphere is non-oxidizing is to suppress the evaporation of Cr at high temperatures, and inert gases such as Ar, He, N2, or reducing gases such as H, CO, CH, etc. Combustion exhaust gas can be used.

<Example> Hereinafter, the present invention will be specifically explained based on Examples.

(Example 1) In order to investigate the influence of the size of MnS in a sintered body on corrosion resistance, machinability, and specularity, as shown in Table 1, MnS was
, four types of steel powder with different compositions of S were produced by spraying with water atomization (Invention Examples 1 to 4). The average particle size of these steel powders was measured using a microtrack and is shown in Table 1, and it was 15 μm.
You can see that it is as follows. In Table 1, Mv represents the volume average diameter, and D50 represents the 50% average diameter. Appropriate amounts of thermoplastic resin, polymer, and paraffin were added as binders to the steel powder in a range of 10 to 15% by weight, and the mixture was kneaded to obtain a compound. Charpy specimens were injection molded using this compound and heated to 60°C at a heating rate of 10°C/h in nitrogen.
The binder was removed by heating to 0°C.

Thereafter, at 1150°C in a vacuum of O, Itorr or less,
The sample was sintered for 2 hours at 1350° C. in Ar.

Corrosion resistance, machinability, and specularity were evaluated using the obtained sintered body. For the corrosion resistance test, 10 Charpy test pieces of 10 mm square were prepared for each steel type, and NaC
j2.After being kept in artificial sweat with a composition of a mixture of urea, ammonia, and lactic acid at a temperature of 40℃ for 24 hours, the presence or absence of rust was examined using a stereomicroscope. Good condition; if even the slightest discoloration was observed, it was considered rust.

In addition, for the machinability test, a Charpy test piece was used to
A test was conducted for drilling holes using a mmφ drill. Cutting conditions: drill rotation speed 3000 rpm, feed 15 mm/min
The drill hole depth was 5 mm, dry cutting was performed, and the machinability was evaluated by the number of six pieces until the drill became unable to cut and broke.

Further, the surface of the Charpy test piece was huff polished, and the specularity of the mirror-finished surface was evaluated in three grades of A, B, and C based on the reading of the scale using a gloss meter. A is 90% or more of the specularity in the ingot material, B is 80-90%, C
has a gloss level of 80% or less.

Table 2 shows the amounts of c, , and o in the sintered body, which are appropriate amounts of C1O. In addition, the same table shows the density ratio of the sintered body measured by the underwater method, which shows that 92% or more was obtained, and the results of measuring the size of MnS in the sintered body using an image processing system are also shown. However, it is 15 μm or less. The same table shows the evaluation results of the corrosion resistance, machinability, and specularity of square steel, and the number of drilled holes increases as the amount of S added increases. In addition, the corrosion resistance of all four steel types was good.
In addition, the specularity of all steel types is of the highest quality.

(Comparative Examples 1 to 5) Compared to Example 1, in order to investigate the influence of the size of MnS in the sintered body on corrosion resistance, machinability, and specularity, steel with the same composition as Example 1 was used. Powders were manufactured (Comparative Examples 1 to 4)
). At that time, MnS, which is a free-cutting component, was added to stone powder using a reagent of 1200#. Further, ordinary 5US316L steel powder was also sprayed and produced (Comparative Example 5). Using this steel powder, molding, degreasing, and sintering were performed under the same conditions as in Example. Table 2 shows c, oit, and sintered body density ratio for each steel type, and appropriate values were obtained. In addition, the same table shows the results of measuring the size of MnS in the sintered body using the same method as in Example 1, and it is found that MnS with a maximum size of 20 to 35 μm exists in all steel types.

The same table shows the results of corrosion resistance, machinability, and specularity evaluated using the same method as in Example. First, the normal 5US316
L has good corrosion resistance and specularity, but has extremely low machinability and requires the addition of free-machining elements. As in Example 1, the machinability improved as the amount of S added increased. Regarding corrosion resistance, all steel types are recognized to rust, and C9
Considering that the amount of 0 is appropriate, it is determined that a large amount of MnS was dissolved in the artificial sweat and the corrosion resistance deteriorated. Furthermore, in terms of specularity, only Comparative Example 1, which has the lowest amount of M n S added, was ranked B, and the remaining ones were ranked C.
It can be seen that increasing the size of nS reduces the specularity.

23 (Example 2) Here, we will discuss the influence of sintered body density.

Water atomized steel powder having the composition and average particle size shown in Table 3 was prepared. A compound was produced using this steel powder in the same manner as in Example 1, and injection molding and degreasing were performed.
Thereafter, sintering was performed at 1150°C under a vacuum of 0.1 torr.
Subsequently, the temperature was maintained in the range of 1150 to 1350°C for 2 hours to obtain sintered bodies with different densities. Using this sintered body, corrosion resistance, machinability, and specularity were evaluated in the same manner as in Example 1.

Table 4 shows the experimental results. Appropriate values were obtained for the amount of C1O for each steel type. Those with a sintered body density ratio of 92% or more (Invention Examples 5 to 7) obtained good results in terms of corrosion resistance, machinability, and specularity, but those with a density ratio of 88.2%,
Rust was observed in the 91.4% samples (Comparative Examples 6 and 7),
The specularity also decreased to B and C ranks, respectively.

From this, the sintered body density needs to be 92% or more, For that purpose, the sintering temperature is 1 50℃ or higher It turns out that it is necessary.

(Example 3) Here, the influence of the amounts of Cr and Ni added on corrosion resistance and specularity will be described. As shown in Table 5, eight types of steel powder with different amounts of Cr and Ni added were sprayed by water atomization to produce the steel powder. Using this steel powder, compound production, injection molding, degreasing and sintering were performed in the same manner as in Example 1. Thereafter, corrosion resistance and specularity were evaluated in the same manner as in Example 1. The experimental results are shown in Table 6, and each sintered body had an appropriate amount of C10 and density ratio. In addition, regarding corrosion resistance and specularity, all steel types have been changed to ones with good specularity of rank A, but the amount of Cr added is 15
% (Comparative Example 8), rusting was observed, and rusting occurred even in the case where the Ni addition amount was 5% (Comparative Example 9). From this, in order to obtain corrosion resistance, Cr, N
It can be seen that each i needs to be added in an appropriate amount.

7 Tokukaihei 3 247743 (11) (Example 4) Here, the effects of adding Se and Te will be described.

As shown in Table 7, steel powder to which Se and Te were added in addition to S was produced by spraying with water atomization. In addition, MnSe and MnT were added to have the same composition as the atomized steel powder.
Steel powder mixed and added in step e was prepared. Using these steel powders, compound production, injection molding, degreasing, and sintering were performed in the same manner as in Example 1. Thereafter, corrosion resistance, machinability, and specularity were evaluated in the same manner as in Example 1. The experimental results are shown in Table 8, and each sintered body has an appropriate C9
The O amount and density ratio have been obtained. In this state, the size of the compound in the sintered body was measured, and the results showed that the size of the compound in the sintered body was smaller than 15 μm in the cases where the pre-alloy was added (Inventive Examples 14 to 19), but it was MnSe.

The powder mixed with M n Te (Comparative Examples 10 to 15) has a maximum of 30 to 40 LLm. Machinability is Se
, Te are improved as the amount added increases. Steel types with prealloy added (invention examples 14 to 19)
shows good results in both corrosion resistance and machinability, but when mixed powders were added (Comparative Examples 10 to 15), rust was observed in all steel types, and specularity was also lower than B and C.
It can be seen that as the size of the compound in the sintered body increases, both corrosion resistance and machinability decrease.

33 (Example 5) Here, the influence of the C10 molar ratio of a free-cutting stainless steel sintered body on corrosion resistance and specularity will be described. Using the free-cutting stainless steel powder shown in Table 9, it was molded under the same conditions as in Example], and then the temperature increase rate was 10"C/min in a nitrogen atmosphere.
The binder was removed by heating to 600°C. The C10 molar ratio at this stage is 33 (Comparative Example 16). Next, about a part of this, 50
C10 adjustment was carried out in two patterns: holding 10 m1n at 0''C (invention example 20) and holding 10 m1n at 350°C in the atmosphere (invention example 21). Thereafter, the above three types of molded bodies were subjected to Example 1. It was sintered under the same conditions as above, and its corrosion resistance and specularity were evaluated.

Table 10 shows the results. The two types with C10 adjustment have an appropriate C10 molar ratio, and as a result, the amount of C1O in the sintered body is reduced, and the corrosion resistance and specularity are also good. However, in Comparative Example 16 in which C10 adjustment was not performed, residual C
It can be seen that both specularity decreases.

<Effects of the Invention> The present invention provides a free-cutting stainless steel sintered body with excellent corrosion resistance and specularity by making the size of the free-cutting compound phase in the sintered body fine. It is.

7

Claims (5)

[Claims] (1) Cr: 16-25% by weight, Ni: 8-24% by weight
, Mo: 10% by weight or less, Mn: 2.0% by weight or less, C
:0.06% by weight or less and 0:0.5% by weight or less, S:0.02~
0.3% by weight, Se: 0.01-0.3% by weight and T
e: A powder sintered body containing one or more free-cutting components from 0.01 to 0.3% by weight, with the remainder consisting of Fe and unavoidable impurities. The component is Mn
Corrosion resistance and machinability characterized by existing as a compound in the form of S, MnSe, and MnTe, the size of the compound is 15 μm or less, and the density of the sintered body is 92% or more of that of the molten body. and sintered alloy steel with excellent specularity. (2) Cr: 16-25% by weight, Ni: 8-24% by weight
, Mo: 10% by weight or less, Mn: 2.0% by weight or less, and S: 0.02~
0.3% by weight, Se: 0.01-0.3% by weight and T
e: Using steel powder containing one or more free-cutting components from 0.01 to 0.3% by weight, the remainder consisting of Fe and unavoidable impurities, and having a particle size of 15 μm or less. After adding and mixing a binder to this steel powder and molding, the binder in the molded body is removed by heating under reduced pressure and/or in a non-oxidizing atmosphere, and then heated at a temperature of 1050 to 1350°C.
Hereinafter, sintered alloy steel with excellent corrosion resistance, machinability, and specularity is characterized by being sintered under reduced pressure of 30 torr or less and further sintered at a temperature of 1250 to 1350°C in a non-oxidizing atmosphere. manufacturing method. (3) Corrosion resistance and machinability according to claim 2, wherein after removal of the binder in the compact and before sintering, the C/O molar ratio in the compact is adjusted to 0.3 to 3.0. and a method for producing sintered alloy steel with excellent specularity. (4) The method for producing a sintered alloy steel with excellent corrosion resistance, machinability, and specularity according to claim 3, wherein the C/O molar ratio in the compact is adjusted by heat treatment under wet hydrogen. . (5) Corrosion resistance according to claim 3, wherein the C/O molar ratio in the molded body is adjusted by heat treatment in the atmosphere;
A method for manufacturing sintered alloy steel with excellent machinability and specularity.